Production of transportation fuels from renewable biomass resources can reduce dependence on traditional fossil fuel, relieve the energy crisis, create new jobs, stimulate local economies, and reduce greenhouse gas emissions. Currently bioethanol is produced from cornstarch or sugarcane. Such processes are not sustainable and are unable to meet the increasing demand for renewable fuels. Sustainable production of biofuel must rely on abundant, inexpensive, and non-food lignocellulosic biomass. A core bottleneck of biomass conversion processes based on a sugar platform is the effective release of sugars from inexpensive, non-food and abundant lignocellulosic biomass at low cost and low energy input[1]. Cellulose in lignocellulosic materials is wrapped by hemicellulose and especially lignin, making cellulose far more difficult to hydrolyze into glucose than starch. In addition cellulose has a crystalline structure and stronger glucosidic bonds than starch (β- vs. α-glucosidic bonds). As a consequence, relatively harsh conditions such as high temperature and more chemicals are needed for hydrolyzing cellulose[2]. Primary methods that have been extensively investigated for saccharification of lignocellulosic materials include concentrated acid, diluted acid, ionic liquid and enzymatic processes.
Concentrated acid saccharification is the most extensively studied cellulose hydrolysis process. This process is conducted at relatively mild temperature and can lead to nearly theoretical yield of sugars. In this process, cellulose in lignocellulosic materials is first swollen at room temperature with concentrated acid (typically sulfuric acid), and then the swollen cellulose is hydrolyzed in diluted acid at elevated temperature (50˜120° C.)[3]. However, acid corrosion of equipment and the difficulty in recycling concentrated sulfuric acid have restricted the development of this technology. Although ion exclusion chromatography can be used to separate sugars and sulfuric acid, the method is costly and energy-intensive. In addition, the acid is extensively diluted during the sugar-acid separation, and of the recovered sulfuric acid has to be reconcentrated to 70%˜80% prior to reuse.[4]
In order to avoid the use of concentrated acid, a saccharification method using diluted acid at higher temperatures (160˜190° C.) was developed. Unfortunately, the dilute acid process only gives a sugar yield of about 50% because of incomplete hydrolysis of cellulose and sugar degradation at high temperature. Additionally, the sugar degradation products, such as furfural, hydroxymethylfurfural (HMF), and levulinic acid, can inhibit the fermentation of the sugars, for example, to produce ethanol. In order to reduce the degradation of sugars, in particular of pentoses, a two-stage process was developed. In a first stage, hemicellulose was first extracted at moderate temperature; and in a second stage, temperature was elevated to hydrolyze cellulose into glucose. Even so, the total yield of sugars was only 60˜70%, depending on feedstock and processing conditions[3b, 5]. Further, the need to employ a two-stage process adds to complexity and cost.
In summary, problems encountered with acid processes include low sugar yield due to the incomplete hydrolysis of cellulose and undesirable degradation of the sugars, formation of fermentation inhibitors (furfural, HMF, and levulinic acid etc.), extensively condensed lignin (which limits the coproducts potential of the lignin), equipment corrosion, acid recovery, and wastewater treatment.
The enzymatic saccharification of lignocellulose using cellulose and hemicellulose hydrolytic enzymes is another popular method used to break down cellulose and hemicellulose into monosaccharides. Enzymatic saccharification itself is inexpensive and less hazardous than acid hydrolysis because of the use of mild process conditions (˜50° C. and pH 4-5). However, enzymatic saccharification of lignocellulosic biomass is economically less attractive which limits its commercialization. A major obstacle to successful commercialization of enzymatic saccharification is the unavailability of high-activity and low-cost enzymes (both cellulases and hemicellulases). Although significant progress has been made in recent decades in improving enzyme activity and reducing enzyme production cost, enzyme is still a considerable contributor to the high cost of the sugars from lignocellulosic biomass[6]. Additionally, because of the natural recalcitrance of lignocellulosic biomass to the enzymes, enzymatic saccharification of untreated raw biomass is very difficult and very slow. In order to achieve a satisfactory level of cellulose hydrolysis, an energy- and cost-intensive pretreatment operation is required. Such pretreatment functions to remove lignin and/or hemicellulose to expose cellulose. Pretreatment can result in destruction of the physical matrix by mechanically grinding or milling to reduce particle size (and thereby increasing accessible surface area to enzymes), enhancing cellulose hydrolysis by decrystallization and depolymerization, or combinations thereof. Representative pretreatment technologies include, for example, acid treatment (e.g., with diluted acid, concentrated phosphoric acid, etc.); the organosolv process (e.g., U.S. Pat. No. 3,585,104), ammonia fiber expansion (AFEX), treatment with ionic liquid, treatment with alkali, and sulfite processes[7]. However, due to technical and/or economic barriers, none of these technologies has as yet commercially succeeded. In addition, unlike chemical reaction, enzymatic hydrolysis is a time-consuming process and typically takes days to complete. Finally, since high consistency (substrate solid content) hydrolysis is an engineering challenge, enzymatic hydrolysis typically generates a dilute (5-10%, w/w) sugar stream.
Recently, direct hydrolysis of lignocellulosic biomass in ionic liquid has been reported from pure cellulose and real biomass, such as untreated corn stover, wheat and rice straws, and wood powder[8, 9]. The use of ionic liquids can be problematic due to the generally higher cost of these materials and to the complexity that can be encountered in separation of the ionic liquids from products and the recycling of ionic liquids.
U.S. Pat. No. 4,018,620 (Penque) relates to a method of hydrolyzing cellulose to mono saccharides by treating cellulose with aqueous CaCl2 and acid, using 55% calcium chloride in the presence of acid to hydrolyze newsprint (newspaper). An overall saccharification yield of 50% was reported, but cellulose was only hydrolyzed by 20%[10c]. Because of its capability of swelling and dissolving cellulose, ZnCl2 is widely used in cellulose solvent systems[11]. A two-step process was reported to hydrolyze cellulose with ZnCl2, swelling and dissolving cellulose at high ZnCl2 concentration followed by hydrolyzing cellulose to glucose at diluted ZnCl2 in the presence of acid[10d]. It was reported that over 90% of pure cellulose could be saccharified to glucose with the process. However, the process was less effective when applied to real lignocellulosic biomass where an overall saccharification yield of polysaccharides (cellulose and hemicellulose) was 60˜70%, but that of cellulose was only 30˜50%.
U.S. Pat. Nos. 4,713,118 and 4,787,939 relate to a process for modification, solubilization and/or hydrolysis of a glycosidically linked carbohydrate having reducing groups. The process employs a mixture of water, an inorganic acid and a halide of lithium, magnesium or calcium.
While processes are known in the art for hydrolyzing lignocellulosic materials there is still a significant need in the art for efficient and low-cost processes which provide hydrolysis of lignocellulosic materials, particularly wood-based materials that are hard to hydrolyze, predominantly to monosaccharides, with minimal loss to undesired coproducts and preferably without the need for pretreatment of lignocellulosic materials.